The purpose of the proposed research is to study the solubilized cell wall components of strains of Streptococcus sanguis, isolated from confirmed cases of bacterial endocarditis (BE), that interact with the surface of human blood platelets. Several studies suggest that the pathogenesis of BE involves, first, the adherence of platelets to altered endothelium. Next platelets would be activated by binding specific bacteria. These events would promote the characteristic deposition of fibrin and subsequent recruitment of polymorphonuclear leukocytes seen in the lesion of BE. Therefore, the specific aims are to: 1. demonstrate that S. samguis will bind to Ca ions poor, inactive blood platelets in the absence of serum; 2. Show that platelet-binding components are present on the cell wall of these microorganisms; 3. isolate, purify and characterize these components biochemically and immunologically; and 4. define the structural residue(s) that specify the microorganism's ability to bind platelets. Bacterial surface components will be solubilized by non-ionic surfactants and fractionated by preparative isoelectric focusing. Fractions of interest will be identified by interaction with platelets as observed spectrophotometrically in an aggregometer and by direct visualization in a microtiter plate assay system. Further purification will be obtained by narrow pH isoelectric focusing and PAGE. The binding components will be characterized by composition, pI, Mr and Immunological properties. The functionally important structural residues will be identified by a competitive ligand assay.